diff --git a/ext/mri/bcrypt_pbkdf_ext.c b/ext/mri/bcrypt_pbkdf_ext.c
index df94a37..3aa628d 100644
--- a/ext/mri/bcrypt_pbkdf_ext.c
+++ b/ext/mri/bcrypt_pbkdf_ext.c
@@ -7,6 +7,9 @@ static VALUE cBCryptPbkdfEngine;
 /* Given a secret and a salt a key and the number of rounds and returns the encrypted secret
 */
 static VALUE bc_crypt_pbkdf(VALUE self, VALUE pass, VALUE salt, VALUE keylen, VALUE rounds) {
+  StringValue(pass);
+  StringValue(salt);
+
   size_t okeylen = NUM2ULONG(keylen);
   if (okeylen == 0 || okeylen > 1024)
     return Qnil;
@@ -14,8 +17,8 @@ static VALUE bc_crypt_pbkdf(VALUE self, VALUE pass, VALUE salt, VALUE keylen, VA
   VALUE out;
 
   int ret = bcrypt_pbkdf(
-    StringValuePtr(pass), RSTRING_LEN(pass),
-    (const u_int8_t*)StringValuePtr(salt), RSTRING_LEN(salt),
+    RSTRING_PTR(pass), RSTRING_LEN(pass),
+    (const u_int8_t*)RSTRING_PTR(salt), RSTRING_LEN(salt),
     okey, okeylen,
     NUM2ULONG(rounds));
   if (ret < 0) {
@@ -29,11 +32,14 @@ static VALUE bc_crypt_pbkdf(VALUE self, VALUE pass, VALUE salt, VALUE keylen, VA
 
 static VALUE bc_crypt_hash(VALUE self, VALUE pass, VALUE salt) {
   u_int8_t hash[BCRYPT_HASHSIZE];
+  StringValue(pass);
+  StringValue(salt);
+
   if (RSTRING_LEN(pass) != 64U)
     return Qnil;
   if (RSTRING_LEN(salt) != 64U)
     return Qnil;
-  bcrypt_hash((u_int8_t*)StringValuePtr(pass), (u_int8_t*)StringValuePtr(salt), hash);
+  bcrypt_hash((u_int8_t*)RSTRING_PTR(pass), (u_int8_t*)RSTRING_PTR(salt), hash);
   return rb_str_new((const char*)hash, sizeof(hash));
 }
 
@@ -45,4 +51,4 @@ void Init_bcrypt_pbkdf_ext(){
 
     rb_define_singleton_method(cBCryptPbkdfEngine, "__bc_crypt_pbkdf", bc_crypt_pbkdf, 4);
     rb_define_singleton_method(cBCryptPbkdfEngine, "__bc_crypt_hash", bc_crypt_hash, 2);
-}
\ No newline at end of file
+}
diff --git a/test/bcrypt_pnkdf/engine_test.rb b/test/bcrypt_pnkdf/engine_test.rb
index 98d990c..2094b99 100644
--- a/test/bcrypt_pnkdf/engine_test.rb
+++ b/test/bcrypt_pnkdf/engine_test.rb
@@ -82,6 +82,17 @@ def test_hash_argument_lengths
     assert_nil BCryptPbkdf::Engine.__bc_crypt_hash(sha2pass, sha2salt.byteslice(0, 63))
   end
 
+  def test_string_arguments_are_validated_before_native_access
+    pass = Object.new
+    pass.define_singleton_method(:to_str) { "pass" }
+    salt = Object.new
+    salt.define_singleton_method(:to_str) { "salt" }
+
+    assert_equal BCryptPbkdf.key("pass", "salt", 32, 4), BCryptPbkdf.key(pass, salt, 32, 4)
+    assert_raises(TypeError) { BCryptPbkdf::Engine.__bc_crypt_hash(false, "s" * 64) }
+    assert_raises(TypeError) { BCryptPbkdf::Engine.__bc_crypt_hash("p" * 64, false) }
+  end
+
   # Issue #31/33: xmalloc(okeylen) was called before the guards inside
   # bcrypt_pbkdf(), so out-of-range keylen caused a heap allocation that was
   # never freed when bcrypt_pbkdf() returned -1. `loop { key("p","s",2000,1) }`